The invention relates to a compressor comprising a housing and at least one rotor rotatably mounted in the housing by means of a shaft, the rotor rotating without contact with the housing.
Compressors generally require to be cooled to dissipate the heat developing during the compression process. A direct cooling of the rotors and shafts is dispensed with in most cases for reasons of cost. Cooling of the rotors is then effected only indirectly via the flow of media conveyed and via the directly cooled housing.
Due to the housing being cooled directly, for instance by an airflow or a water cooling jacket, and the rotors being cooled only indirectly, a high temperature difference occurs in operation between the housing and the rotors. This temperature difference needs to be taken into consideration in dimensioning the gaps. The larger temperature expansion of the rotors is allowed for by enlarged gaps in the cold condition. The difference between the gap size in the cold condition and the gap size in the operating condition, i.e. with a temperature difference in the order of 100° K, is referred to as gap reduction. In order to prevent the rotors from striking against the housing at all events, the gap widths are defined to allow for the maximum thermal stress as results from the varying pressure ratios and speeds. Taking the gap reduction into account then leads to a dimensioning of the gap widths in the cold condition. Efforts are made however to keep the gaps as small as possible so as to minimize backflows and maximize both the volumetric and the isentropic efficiency.
In practice, these considerations result in the use of materials featuring low thermal expansion. The standard materials employed are lamellar graphite cast iron for the housing and nodular graphite cast iron for the rotors. The coefficient of thermal expansion is αk=10.5−6/K in both cases. When cast iron is used for the housing and the rotors and when the rotors have an outer diameter of 100 mm, for example, a value of approximately 0.1 mm results for the gap reduction. This is sufficient to achieve satisfactory efficiencies. Use of a material such as aluminum, on the other hand, is out of the question since owing to the thermal expansion, which is more than twice as large, the corresponding values of the gap reduction would be in the range of about 0.24 mm, so that in the cold condition the gap widths would have to be more than twice as large, which would result in an enormous increase in gap leakages.